Oil and gas wells are completed with a string of tubular steel casing which is cemented in place. When properly placed and cemented in the well bore, the casing protects freshwater reservoirs from contamination, supports unconsolidated rock formations, maintains natural separation of formations, aids in the prevention of blowouts and waste of reservoir energy, and acts as a conduit for receiving production tubing through which formation fluids are brought to the surface.
In some wells, the well casing intersects multiple producing formations which are confined vertically between multiple layers of underburden and overburden, typically of impervious siltstone or other barren rock. Consequently, each producing formation must be produced separately. To make this possible, the well casing must be perforated at separate locations which coincide with the separate producing zones. Additionally, the different producing zones must be isolated with respect to each other by packers installed within the casing bore below and above each producing zone.
Gun perforation is a method for providing a well bore reservoir connection by forming holes through the casing and into the surrounding formation. Two conventional perforating methods are bullet perforating and jet perforating. In both methods, the gun is fitted with barrels which contain the perforating apparatus. Each barrel is wired for firing by remote control from the surface. The gun is run into the well bore on a wire line from a service truck. The wire line lowers and raises the gun in and out of the hole, and when the gun is in position to be fired, the operator sends an electric impulse down the wire line to trigger detonation. The bullet or perforating jet punches a hole through the steel casing, the cement sheath, and several inches into the surrounding reservoir, thus creating a flow passage from the reservoir into the well bore. Guns of the bullet type are retrievable and are reloaded at the surface. Many jet type guns are expendable and disintegrate upon detonation, although some types are retrieved and either reloaded or discarded.
After the perforations have been formed, well completion continues with the installation of production tubing, isolation packers, a safety valve, and circulation tools such as sliding side door circulation valves for gas wells, and screens for oil wells.